The basic premise in the understanding of our origins, and the properties of
all the planets we have studied this term, is that natural forces created
and shaped the Solar System. And that there is a continuity to that
process, i.e. it is not a sequence of random events.

Any model or theory for the formation of the Solar System must have a
set of explanations for large-scale and small-scale properties.

Large-Scale:

the planets are isolated in orderly intervals

orbits are nearly circular

orbits are in the same plane

all planets revolve prograde

Small-Scale:

most planets rotate prograde

the systems of moons can be divided into regular objects
(spherical) with direct orbits versus irregular objects with eccentric
orbits

terrestrial planets have

high densities

thin or no atmospheres

rotate slowly

rocky, poor in ices and H/He

jovian worlds have

low densities

thick atmospheres

rotate rapidly

many moons

fluid interiors, rich in ices, H/He

most of outer SS objects (not just jovian worlds) are ice-rich

Also note that the overall architecture of our Solar System is orderly
and the ages of its members uniform. All indicators point to a single
formation event about 4.6 billion years ago.

The above is not to ignore the fact that a great deal of evolution
occurred in the Solar System after it formed (see below). For example, the
origin secondary atmospheres of the terrestrial worlds underwent a large
amount of chemical processing (Venus was baked, Mars was frozen, Earth
developed life). There was also orbital evolution as well, rings were
formed, moons captured, tidal locking between worlds (e.g. Pluto and
Charon). So the Solar System is not a static system, it is dynamic.

How does one test a hypothesis?

To answer scientific questions requires the formulation of a hypothesis.
The hypothesis is tested against the facts to look for contradictions
that rule out or require modification to the hypothesis. Note that
the process of hypothesis formulation and then theory building is a
lengthy, career dependent operation. So the sociology of science
requires that a hypothesis be tested and confirmed by many scientists
since the creator of the hypothesis has a strong psychological
attachment to his work.

Encounter Hypothesis:

One of the earliest theories for the formation of the planets was called
the encounter hypothesis. In this scenario, a rogue star passes close
to the Sun about 5 billion years ago. Material, in the form of hot gas,
is tidally stripped from the Sun and the rogue star. This material
fragments into smaller lumps which form the planets. This hypothesis
has the advantage of explaining why the planets all revolve in the same
direction (from the encounter geometry) and also provides an
explanation for why the inner worlds are denser than the outer
worlds.

However, there are two major problems for a theory of this type. One is
that hot gas expands, not contracts. So lumps of hot gas would not form
planets. The second is that encounters between stars are extremely
rare, so rare as to be improbable in the lifetime of the Universe (15
billion years).

Nebular Hypothesis:

A second theory is called the nebular hypothesis. In this theory, the
whole Solar System starts as a large cloud of gas that contracts
under self-gravity. Conservation of angular momentum
requires that a rotating disk form with a large concentration at the
center (the proto-Sun). Within the disk, planets form.

While this theory incorporates more basic physics, there are several
unsolved problems. For example, a majority of the angular momentum in
the Solar System is held by the outer planets. For comparison, 99% of
the Solar System's mass is in the Sun, but 99% of its angular momentum
is in the planets. Another flaw is the mechanism from which the disk
turns into individual planets.

Protoplanet Hypothesis:

The current working model for the formation of the Solar System is called
the protoplanet hypothesis. It
incorporates many of the components of the nebular hypothesis, but adds
some new aspects from modern knowledge of fluids and states of matter.

Meanwhile in the inner Solar System:

Note that as the planet's began to form they grew in mass by accreting
planetesimals. Since force of gravity is proportional to mass, the
largest planetesimals are accreted first. The early proto-planets are
able to sweep the early Solar System clean of large bodies.

Notice also that the lighter compounds are vaporized in the inner Solar
System. So where did all the outgassing material come from? The
answer is comets that fall from the outer Solar System after the planets
form.

Meanwhile in the outer Solar System:

The Jovian worlds, having an early edge on gathering mass in the
colder outer solar disk, were the most efficient at capturing
planetesimals, which only served to increase their already large
masses. As the planetesimals shrink in average size, collisions with
proto-planets lead to fragmentation. So quickly the Solar System
divided into large proto-planets and smaller and smaller
planetesimals which eventually became the numerous meteors we see
today.

Any leftover large bodies were captured as moons or ejected by
gravity assist into the Oort cloud. The start of thermonuclear
fusion in the Sun's core created enough luminosity so that the
remaining hydrogen and helium gas in the solar disk was removed by
radiation pressure.

The only remaining problem is the distribution of angular momentum. The
current explanation for the fact that most of the angular momentum is
in the outer planets is that, by some mechanism, the Sun has lost angular
momentum. The mechanism of choice is magnetic braking.

The early Sun had a much heavier flow of solar winds particles. Many
of the particles in the solar wind are charged, and are effected by
the laws of motion as well as electromagnetic forces. As the solar
wind leaves the solar surface, they are ``dragged'' by the magnetic
field, which in turn slows down the Sun's rotation.

Migrating Planets:

The protoplanet hypothesis explains most of the features of the Solar System;
however, the outer solar system is still strange, especially the properties of
Pluto/Charon. One explanation is that the Solar System was not born in the
configuration that we see today. That the planets in the outer Solar System
migrated to their present positions.

Migration requires some interaction between the planet and a fairly large body or
the gravitational forces are too weak. Early in the formation of the Solar System,
there were lots of Moon-sized to Mars-sized bodies, especially in the outer SS. A
large planetesimal that crosses near Neptune will lose some energy, fall down near
Jupiter, gain energy to be ejected into the Oort Cloud.

This will have the effect of decreasing the size of Jupiter's orbit, and expanding
the size of Saturn, Uranus and Neptunes' orbits. As Neptune moves outward, it will
beginning to perturb the orbits of the trans-Neptunian objects (large ice covered
astroids of which Pluto/Charon are a member). This pushes Pluto/Charon into a
highly eccentric, inclined 3:2 resonant orbit that it occupies today.

All the leftover planetesimals near Neptunes orbit are pushed into a torus shaped
region called the Kuiper belt. Smaller planetesimals are thrown farther out into
the Oort cloud.

Extrasolar Systems:

Support for the protoplanet hypothesis has been found by the detection of
disk material around of stars, such as Beta Pictoris and by Hubble images
of the Orion Nebula.

There are now numerous verified solar type stars that have Jupiter size
planets in orbit around them.

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Doppler Detection:

Doppler spectroscopy is used to detect the periodic velocity shift of the
stellar spectrum caused by an orbiting giant planet. This method is also
referred to as the radial velocity method. From ground-based
observatories, spectroscopists can measure Doppler shifts greater than 3
m/sec due to the reflex motion of the star This corresponds to a minimum
detectable mass of 33 Earth masses for a planet at 1 AU from a one solar-mass
star.

All stars exhibit some Doppler shift because all stars are moving in the
gravitational field of our Galaxy. But the stars that move toward us, then
away from us, then toward us again wobbling with periodic motion are
gravitationally bound to another object. The magnitude of this changing
velocity, together with the period of the motion, reveals the mass of an
invisible companion to a visible star. Our Sun moves with a speed of about
12 meters/sec due to the most massive planet in our Solar System, Jupiter.
The periodicity of the Sun's motion is the same as the orbital period of
Jupiter: one cycle takes about twelve years.

Transit Method:

A more direct measure occurs is a planet transits across the front of a
star, as happened with HD 209458, a distant planet passing in front of its
star, providing direct and independent confirmation of the existence of
extrasolar planets.

Astronomers predicted the planet would cross the face of the star if the
planet's orbital plane were lucky enough to carry it between Earth and the
star. Until now, none of the 18 other extrasolar planets discovered has
had its orbital plane oriented edge-on to Earth so that the planet could
be seen to transit the star, nor have any of the other planets discovered
by other researchers. However, on Nov. 7, 1999 an automatic telescope
observed a 1.7 percent dip in the star's brightness.

With the orbital plane of the planet known, the astronomers for the first
time could determine precisely the mass of the planet and, from the size
of the planet measured during transit, its density. Interestingly, while
the planet's mass is only 63 percent of Jupiter's mass, its radius is 60
percent bigger than that of Jupiter. This fits with theories that predict
a bloated planet when, as here, the planet is very close to the star.

Habitable Zone:

One of the main ingredients for life as we know it is liquid water. Water
exists as a liquid between 273K and 373K (unless the pressure is too low,
in which case the water sublimates into gaseous water vapor). The region
on the solar system (or any planetary system) where the temperature is in
this range, is called the habitable zone.

Planets are in equilibrium with their surroundings: they are neither
getting hotter nor colder. All planets absorb incident radiation from the
Sun (this heats them up); to maintain equilibrium, they must radiate away
the same amount of energy. The temperature of a planet can be approximated
by assuming that it is a black body.

Planets do not absorb all incident light; much gets reflected. The albedo
is the fraction of incident light reflected, not absorbed. The albedo of
the Earth is 0.37; that of Venus is 0.65; that of the Moon is about 0.12
(clouds are highly reflective, basaltic rock is not). You must multiply the
solar irradiance by the albedo. This extends the inner edge of the
habitable zone inwards

Another complication is that planets are not ideal black bodies. Carbon
dioxide, water vapor, and other atmospheric gases are opaque in the
near-IR (where the peak of the black body emission would be). A
less-than-ideal radiator must be hotter than a black body to radiate the
same amount of luminosity. This extends the outer edge of the habitable
zone outwards.